Energy quenchers

Chart 9.12 Chemical structures of typical nickel chelates used as quenchers in polyalkenes [93]. 

The importance of quenchers derives mainly from their ability to interact with excited carbonyl groups, which are present in many thermoplastics, especially in polyalkenes. Commercially available energy quenchers include complexes and chelates of transition metals, such as those shown in Chart 9.12. 

It may be the case that energy quenchers also act as UVAs, i.e. that they also protect the polymer by light absorption. 

Chart 9.13 Chemical structures of typical commercial hindered amine stabilizers [109]. 

The macroradicals P generated in this process can initiate oxidative chain reactions and thus reduce the stabilizing power of hindered amines. 

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Thus, the photo-activity of poly[bis(4-benzoylphenoxy)phosphazene] under illumination could be finely tuned by irradiating the polymer in the presence of variable amount of naphthalene, a typical triplet state energy quencher [474]. The same polymer could be used as polymeric photosensitizer to induce the... 

The assignment of the lutein absorbing at 495 nm as lutein 1 has helped with the identification of an excitation energy quencher in LHCII, when the complex is in aggregated form. 

Photochemical reactions in the solid state may also go to limited conversion when the product does not compete for incident light but may act as an efficient energy quencher. This is likely to be of importance in triplet state reactions when the product may act as an energy trap. Although this is a factor that may also limit the conversion of photochemical reactions in solution, because of efficient energy delocalization by dipole-dipole and exchange mechanisms it is likely to be more important in the crystalline solid state. 

Figure 11 Structures of suitable triplet energy quenchers tor butylmethoxy-dibenzoylmethane. Note that all have a double bond allowing for E/Z isomerization. In octocrylene and ParsoP SLX, this Isomerization is for symmetry reasons degenerate and therefore not detectable.

The T2 state lifetime was also estimated for CHR(T2). By combining the estimated lifetime (45 ps) and the ENT rate from the T2 state to a triplet energy quencher, the reaction distance of the bimolecular ENT was estimated to be 3.8 and 3.7 A in cyclohexane and acetonitrile, respectively, indicating that the collision between CHR(T2) and quenchers occurs in accordance with the exchange mechanism [104]. 

Scheme 9.11 Schematic illustration of the action of energy quenchers.

Energy quenchers that react with activated polypropylene to prevent decomposition and then degrade themselves to harmless products... 

The ability of these energy quenchers to stabilize polypropylene fibers to weathering permitted the development of many new end-uses, but their capabilities have been surpassed by a new group of stabilizers that contain a hindered piperidine structure. As shown above, these HALS compounds can be very good long-term thermal stabilizers. Hindered piperidines react with hydroperoxides during polypropylene processing to form nitroxyl radicals (II) that arc effective polymer radical traps [134]. These nitroxyl radicals react with polymer free radicals to form the polymeric hydroxylamine (III). 

Nickel complexes used as energy quenchers to provide UV stability can provide dye sites with some dyes designed to form chelate complexes [146]. In recent years, hydrophihc dyestuff with long alkyl graft chain has been developed, but it can only provide light colors. Several kinds of metal salts such as nickel stearate and zinc stearate are added before fiber formation to... 

Properties Lt. gm. powd. m.w. 572.54 m.p. 258-281 C Toxicology LD50 (oral, rat) >10 g/kg, (dermal, rabbit) >10 g/kg Uses UV and heat stabilizer for polyolefins, ABS protects syn. resins and paints, esp. in aerospace industry energy quencher free radical scavenger... 

Quenchers. When a polymer absorbs UV energy, it may be able to dispose of it harmlessly by intermolecular transfer to certain additives that can then carry the energy away and dispose of it harmlessly. These additives are referred to as energy quenchers. Or-... 

In principle, the difference between the UV absorbers and energy quenchers consists of the protection of absorbers in polymer by absorbing UV light, while the quenchers deactivate the excited states of the polymer molecules. In reality, it was shown that the absorbers can function also as quenchers [1]. 

Cytec s family of light stabilizers includes Cyasorb UV absorbers, which prevent photo-degradation of polymers by competitive UV light absorption, energy quenchers, radical scavengers, and hydroperoxide decomposers. 

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